This describes the protocol used by OpenSSH's ssh-agent.
OpenSSH's agent supports managing keys for the standard SSH protocol
2 as well as the legacy SSH protocol 1. Support for these key types
is almost completely disjoint - in all but a few cases, operations on
protocol 2 keys cannot see or affect protocol 1 keys and vice-versa.
Protocol 1 and protocol 2 keys are separated because of the differing
cryptographic usage: protocol 1 private RSA keys are used to decrypt
challenges that were encrypted with the corresponding public key,
whereas protocol 2 RSA private keys are used to sign challenges with
a private key for verification with the corresponding public key. It
is considered unsound practice to use the same key for signing and
encryption.
With a couple of exceptions, the protocol message names used in this
document indicate which type of key the message relates to. SSH_*
messages refer to protocol 1 keys only. SSH2_* messages refer to
protocol 2 keys. Furthermore, the names also indicate whether the
message is a request to the agent (*_AGENTC_*) or a reply from the
agent (*_AGENT_*). Section 3 below contains the mapping of the
protocol message names to their integer values.
1. Data types
Because of support for legacy SSH protocol 1 keys, OpenSSH's agent
protocol makes use of some data types not defined in RFC 4251.
1.1 uint16
The "uint16" data type is a simple MSB-first 16 bit unsigned integer
encoded in two bytes.
1.2 mpint1
The "mpint1" type represents an arbitrary precision integer (bignum).
Its format is as follows:
uint16 bits
byte[(bits + 7) / 8] bignum
"bignum" contains an unsigned arbitrary precision integer encoded as
eight bits per byte in big-endian (MSB first) format.
Note the difference between the "mpint1" encoding and the "mpint"
encoding defined in RFC 4251. Also note that the length of the encoded
integer is specified in bits, not bytes and that the byte length of
the integer must be calculated by rounding up the number of bits to the
nearest eight.
2. Protocol Messages
All protocol messages are prefixed with their length in bytes, encoded
as a 32 bit unsigned integer. Specifically:
uint32 message_length
byte[message_length] message
The following message descriptions refer only to the content the
"message" field.
2.1 Generic server responses
The following generic messages may be sent by the server in response to
requests from the client. On success the agent may reply either with:
byte SSH_AGENT_SUCCESS
or a request-specific success message.
On failure, the agent may reply with:
byte SSH_AGENT_FAILURE
SSH_AGENT_FAILURE messages are also sent in reply to unknown request
types.
2.2 Adding keys to the agent
Keys are added to the agent using the SSH_AGENTC_ADD_RSA_IDENTITY and
SSH2_AGENTC_ADD_IDENTITY requests for protocol 1 and protocol 2 keys
respectively.
Two variants of these requests are SSH_AGENTC_ADD_RSA_ID_CONSTRAINED
and SSH2_AGENTC_ADD_ID_CONSTRAINED - these add keys with optional
"constraints" on their usage.
OpenSSH may be built with support for keys hosted on a smartcard
or other hardware security module. These keys may be added
to the agent using the SSH_AGENTC_ADD_SMARTCARD_KEY and
SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED requests.
2.2.1 Key constraints
The OpenSSH agent supports some basic optional constraints on key usage.
At present there are two constraints defined.
The first constraint limits the validity duration of a key. It is
encoded as:
byte SSH_AGENT_CONSTRAIN_LIFETIME
uint32 seconds
Where "seconds" contains the number of seconds that the key shall remain
valid measured from the moment that the agent receives it. After the
validity period has expired, OpenSSH's agent will erase these keys from
memory.
The second constraint requires the agent to seek explicit user
confirmation before performing private key operations with the loaded
key. This constraint is encoded as:
byte SSH_AGENT_CONSTRAIN_CONFIRM
Zero or more constraints may be specified when adding a key with one
of the *_CONSTRAINED requests. Multiple constraints are appended
consecutively to the end of the request:
byte constraint1_type
.... constraint1_data
byte constraint2_type
.... constraint2_data
....
byte constraintN_type
.... constraintN_data
Such a sequence of zero or more constraints will be referred to below
as "constraint[]". Agents may determine whether there are constraints
by checking whether additional data exists in the "add key" request
after the key data itself. OpenSSH will refuse to add a key if it
contains unknown constraints.
2.2.2 Add protocol 1 key
A client may add a protocol 1 key to an agent with the following
request:
byte SSH_AGENTC_ADD_RSA_IDENTITY or
SSH_AGENTC_ADD_RSA_ID_CONSTRAINED
uint32 ignored
mpint1 rsa_n
mpint1 rsa_e
mpint1 rsa_d
mpint1 rsa_iqmp
mpint1 rsa_q
mpint1 rsa_p
string key_comment
constraint[] key_constraints
Note that there is some redundancy in the key parameters; a key could be
fully specified using just rsa_q, rsa_p and rsa_e at the cost of extra
computation.
"key_constraints" may only be present if the request type is
SSH_AGENTC_ADD_RSA_IDENTITY.
The agent will reply with a SSH_AGENT_SUCCESS if the key has been
successfully added or a SSH_AGENT_FAILURE if an error occurred.
2.2.3 Add protocol 2 key
The OpenSSH agent supports DSA and RSA keys for protocol 2. DSA keys may
be added using the following request
byte SSH2_AGENTC_ADD_IDENTITY or
SSH2_AGENTC_ADD_ID_CONSTRAINED
string "ssh-dss"
mpint dsa_p
mpint dsa_q
mpint dsa_g
mpint dsa_public_key
mpint dsa_private_key
string key_comment
constraint[] key_constraints
DSA certificates may be added with:
byte SSH2_AGENTC_ADD_IDENTITY or
SSH2_AGENTC_ADD_ID_CONSTRAINED
string "ssh-dss-cert-v00@openssh.com"
string certificate
mpint dsa_private_key
string key_comment
constraint[] key_constraints
RSA keys may be added with this request:
byte SSH2_AGENTC_ADD_IDENTITY or
SSH2_AGENTC_ADD_ID_CONSTRAINED
string "ssh-rsa"
mpint rsa_n
mpint rsa_e
mpint rsa_d
mpint rsa_iqmp
mpint rsa_p
mpint rsa_q
string key_comment
constraint[] key_constraints
RSA certificates may be added with this request:
byte SSH2_AGENTC_ADD_IDENTITY or
SSH2_AGENTC_ADD_ID_CONSTRAINED
string "ssh-rsa-cert-v00@openssh.com"
string certificate
mpint rsa_d
mpint rsa_iqmp
mpint rsa_p
mpint rsa_q
string key_comment
constraint[] key_constraints
Note that the 'rsa_p' and 'rsa_q' parameters are sent in the reverse
order to the protocol 1 add keys message. As with the corresponding
protocol 1 "add key" request, the private key is overspecified to avoid
redundant processing.
For both DSA and RSA key add requests, "key_constraints" may only be
present if the request type is SSH2_AGENTC_ADD_ID_CONSTRAINED.
The agent will reply with a SSH_AGENT_SUCCESS if the key has been
successfully added or a SSH_AGENT_FAILURE if an error occurred.
2.2.4 Loading keys from a smartcard
The OpenSSH agent may have optional smartcard support built in to it. If
so, it supports an operation to load keys from a smartcard. Technically,
only the public components of the keys are loaded into the agent so
this operation really arranges for future private key operations to be
delegated to the smartcard.
byte SSH_AGENTC_ADD_SMARTCARD_KEY or
SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED
string reader_id
string pin
constraint[] key_constraints
"reader_id" is an identifier to a smartcard reader and "pin"
is a PIN or passphrase used to unlock the private key(s) on the
device. "key_constraints" may only be present if the request type is
SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED.
This operation may load all SSH keys that are unlocked using the
"pin" on the specified reader. The type of key loaded (protocol 1
or protocol 2) will be specified by the smartcard itself, it is not
client-specified.
The agent will reply with a SSH_AGENT_SUCCESS if one or more keys have
been successfully loaded or a SSH_AGENT_FAILURE if an error occurred.
The agent will also return SSH_AGENT_FAILURE if it does not support
smartcards.
2.3 Removing multiple keys
A client may request that an agent delete all protocol 1 keys using the
following request:
byte SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES
This message requests the deletion of all protocol 2 keys:
byte SSH2_AGENTC_REMOVE_ALL_IDENTITIES
On success, the agent will delete all keys of the requested type and
reply with a SSH_AGENT_SUCCESS message. If an error occurred, the agent
will reply with SSH_AGENT_FAILURE.
Note that, to delete all keys (both protocol 1 and 2), a client
must send both a SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES and a
SSH2_AGENTC_REMOVE_ALL_IDENTITIES request.
2.4 Removing specific keys
2.4.1 Removing a protocol 1 key
Removal of a protocol 1 key may be requested with the following message:
byte SSH_AGENTC_REMOVE_RSA_IDENTITY
uint32 key_bits
mpint1 rsa_e
mpint1 rsa_n
Note that key_bits is strictly redundant, as it may be inferred by the
length of rsa_n.
The agent will delete any private key matching the specified public key
and return SSH_AGENT_SUCCESS. If no such key was found, the agent will
return SSH_AGENT_FAILURE.
2.4.2 Removing a protocol 2 key
Protocol 2 keys may be removed with the following request:
byte SSH2_AGENTC_REMOVE_IDENTITY
string key_blob
Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key
Algorithms" for either of the supported key types: "ssh-dss" or
"ssh-rsa".
The agent will delete any private key matching the specified public key
and return SSH_AGENT_SUCCESS. If no such key was found, the agent will
return SSH_AGENT_FAILURE.
2.4.3 Removing keys loaded from a smartcard
A client may request that a server remove one or more smartcard-hosted
keys using this message:
byte SSH_AGENTC_REMOVE_SMARTCARD_KEY
string reader_id
string pin
"reader_id" the an identifier to a smartcard reader and "pin" is a PIN
or passphrase used to unlock the private key(s) on the device.
When this message is received, and if the agent supports
smartcard-hosted keys, it will delete all keys that are hosted on the
specified smartcard that may be accessed with the given "pin".
The agent will reply with a SSH_AGENT_SUCCESS if one or more keys have
been successfully removed or a SSH_AGENT_FAILURE if an error occurred.
The agent will also return SSH_AGENT_FAILURE if it does not support
smartcards.
2.5 Requesting a list of known keys
An agent may be requested to list which keys it holds. Different
requests exist for protocol 1 and protocol 2 keys.
2.5.1 Requesting a list of protocol 1 keys
To request a list of protocol 1 keys that are held in the agent, a
client may send the following message:
byte SSH_AGENTC_REQUEST_RSA_IDENTITIES
The agent will reply with the following message:
byte SSH_AGENT_RSA_IDENTITIES_ANSWER
uint32 num_keys
Followed by zero or more consecutive keys, encoded as:
uint32 bits
mpint1 rsa_e
mpint1 rsa_n
string key_comment
2.5.2 Requesting a list of protocol 2 keys
A client may send the following message to request a list of
protocol 2 keys that are stored in the agent:
byte SSH2_AGENTC_REQUEST_IDENTITIES
The agent will reply with the following message header:
byte SSH2_AGENT_IDENTITIES_ANSWER
uint32 num_keys
Followed by zero or more consecutive keys, encoded as:
string key_blob
string key_comment
Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key
Algorithms" for either of the supported key types: "ssh-dss" or
"ssh-rsa".
2.6 Private key operations
The purpose of the agent is to perform private key operations, such as
signing and encryption without requiring a passphrase to unlock the
key and without allowing the private key itself to be exposed. There
are separate requests for the protocol 1 and protocol 2 private key
operations.
2.6.1 Protocol 1 private key challenge
The private key operation used in version 1 of the SSH protocol is
decrypting a challenge that has been encrypted with a public key.
It may be requested using this message:
byte SSH_AGENTC_RSA_CHALLENGE
uint32 ignored
mpint1 rsa_e
mpint1 rsa_n
mpint1 encrypted_challenge
byte[16] session_id
uint32 response_type /* must be 1 */
"rsa_e" and "rsa_n" are used to identify which private key to use.
"encrypted_challenge" is a challenge blob that has (presumably)
been encrypted with the public key and must be in the range
1 <= encrypted_challenge < 2^256. "session_id" is the SSH protocol 1
session ID (computed from the server host key, the server semi-ephemeral
key and the session cookie).
"ignored" and "response_type" exist for compatibility with legacy
implementations. "response_type" must be equal to 1; other response
types are not supported.
On receiving this request, the server decrypts the "encrypted_challenge"
using the private key matching the supplied (rsa_e, rsa_n) values. For
the response derivation, the decrypted challenge is represented as an
unsigned, big-endian integer encoded in a 32 byte buffer (i.e. values
smaller than 2^248 will have leading 0 bytes).
The response value is then calculated as:
response = MD5(decrypted_challenge || session_id)
and returned in the following message
byte SSH_AGENT_RSA_RESPONSE
byte[16] response
If the agent cannot find the key specified by the supplied (rsa_e,
rsa_n) then it will return SSH_AGENT_FAILURE.
2.6.2 Protocol 2 private key signature request
A client may use the following message to request signing of data using
a protocol 2 key:
byte SSH2_AGENTC_SIGN_REQUEST
string key_blob
string data
uint32 flags
Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key
Algorithms" for either of the supported key types: "ssh-dss" or
"ssh-rsa". "flags" is a bit-mask, but at present only one possible value
is defined (see below for its meaning):
SSH_AGENT_OLD_SIGNATURE 1
Upon receiving this request, the agent will look up the private key that
corresponds to the public key contained in key_blob. It will use this
private key to sign the "data" and produce a signature blob using the
key type-specific method described in RFC 4253 section 6.6 "Public Key
Algorithms".
An exception to this is for "ssh-dss" keys where the "flags" word
contains the value SSH_AGENT_OLD_SIGNATURE. In this case, a legacy
signature encoding is used in lieu of the standard one. In this case,
the DSA signature blob is encoded as:
byte[40] signature
The signature will be returned in the response message:
byte SSH2_AGENT_SIGN_RESPONSE
string signature_blob
If the agent cannot find the key specified by the supplied key_blob then
it will return SSH_AGENT_FAILURE.
2.7 Locking or unlocking an agent
The agent supports temporary locking with a passphrase to suspend
processing of sensitive operations until it has been unlocked with the
same passphrase. To lock an agent, a client send the following request:
byte SSH_AGENTC_LOCK
string passphrase
Upon receipt of this message and if the agent is not already locked,
it will suspend processing requests and return a SSH_AGENT_SUCCESS
reply. If the agent is already locked, it will return SSH_AGENT_FAILURE.
While locked, the agent will refuse all requests except
SSH_AGENTC_UNLOCK, SSH_AGENTC_REQUEST_RSA_IDENTITIES and
SSH2_AGENTC_REQUEST_IDENTITIES. The "request identities" requests are
treated specially by a locked agent: it will always return an empty list
of keys.
To unlock an agent, a client may request:
byte SSH_AGENTC_UNLOCK
string passphrase
If the passphrase matches and the agent is locked, then it will resume
processing all requests and return SSH_AGENT_SUCCESS. If the agent
is not locked or the passphrase does not match then it will return
SSH_AGENT_FAILURE.
Locking and unlocking affects both protocol 1 and protocol 2 keys.
3. Protocol message numbers
3.1 Requests from client to agent for protocol 1 key operations
SSH_AGENTC_REQUEST_RSA_IDENTITIES 1
SSH_AGENTC_RSA_CHALLENGE 3
SSH_AGENTC_ADD_RSA_IDENTITY 7
SSH_AGENTC_REMOVE_RSA_IDENTITY 8
SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES 9
SSH_AGENTC_ADD_RSA_ID_CONSTRAINED 24
3.2 Requests from client to agent for protocol 2 key operations
SSH2_AGENTC_REQUEST_IDENTITIES 11
SSH2_AGENTC_SIGN_REQUEST 13
SSH2_AGENTC_ADD_IDENTITY 17
SSH2_AGENTC_REMOVE_IDENTITY 18
SSH2_AGENTC_REMOVE_ALL_IDENTITIES 19
SSH2_AGENTC_ADD_ID_CONSTRAINED 25
3.3 Key-type independent requests from client to agent
SSH_AGENTC_ADD_SMARTCARD_KEY 20
SSH_AGENTC_REMOVE_SMARTCARD_KEY 21
SSH_AGENTC_LOCK 22
SSH_AGENTC_UNLOCK 23
SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED 26
3.4 Generic replies from agent to client
SSH_AGENT_FAILURE 5
SSH_AGENT_SUCCESS 6
3.5 Replies from agent to client for protocol 1 key operations
SSH_AGENT_RSA_IDENTITIES_ANSWER 2
SSH_AGENT_RSA_RESPONSE 4
3.6 Replies from agent to client for protocol 2 key operations
SSH2_AGENT_IDENTITIES_ANSWER 12
SSH2_AGENT_SIGN_RESPONSE 14
3.7 Key constraint identifiers
SSH_AGENT_CONSTRAIN_LIFETIME 1
SSH_AGENT_CONSTRAIN_CONFIRM 2
$OpenBSD: PROTOCOL.agent,v 1.5 2010/02/26 20:29:54 djm Exp $